This invention relates to polyimide copolymers and, more particularly, to low modulus, solvent resistant, wet-etchable, polyimides containing 4,4'-bis(p-aminophenoxy)biphenyl (APBP) and siloxane diamine moieties.
Aromatic polyimides and polyamide-imides have found extensive use in industry as fibers, composites, molded parts and dielectrics due to their toughness, flexibility, mechanical strength and high thermal stability. In the electronic industry, polyimides have proven to be useful due to their low dielectric constant and high electrical resistivity. Such polymers have been used as both films and coatings for insulation, die attach adhesives, flexible circuit substrates, and the like.
In particular, polyimides are useful for interlevel dielectric and passivation applications, for example, as a passivation coating over a substrate containing an integrated circuit. As a passivation coating, the polymer protects the integrated circuit against damage during assembly and provides stress relief between the pressure sensitive areas of a substrate, or an integrated circuit, and the packaging material which encapsulates the integrated circuit assembly. Further, the passivation coating acts as a barrier to ion transport between the outside environment and the integrated circuit and, when the integrated circuit is packaged in epoxy packaging materials, the passivation coating acts as a barrier to alpha particles emanating from the epoxy packaging material. The stress relief capability of a polymer (i.e., the pliability) may be measured as the modulus (also referred to as tensile modulus) of the polymer. A low modulus polymer is preferred because a low modulus polymer will transmit less stress from the exterior of the package to the highly sensitive substrate than would be transmitted by a higher modulus polymer coating.
Siloxane diamine monomers have been used to lower the modulus of a polyimide. However, processing characteristics such as resistance to photoresist solvents and developing solutions are generally sacrificed. As is explained further below, polymers used for microelectronic applications must possess good processing characteristics.
Processing polyimides, for example, in the case of polyimide coated integrated circuits, requires a multistep procedure. Polyimides are generally made by dissolving a diamine in a solvent and then adding a dianhydride to form a solution of polyamic acid. The resulting solution of polyamic acid is spread on a substrate to form a coating and heated (soft-baked) to remove the solvent. The polyamic acid coating is further coated with a photoresist material which itself is in a solvent, and that solvent is removed, typically by heating (also called soft-baking). The photoresist material is then shielded with a mask containing a pattern of openings, and the photoresist material is exposed to actinic radiation. Thus, the photoresist material is photochemically altered such that the areas that were exposed to actinic radiation change solubility, and vias (or openings) are created by taking advantage of this selective solubility to develop and remove specific areas of photoresist material. The polyamic acid coating can then be etched either along with the development and removal of the photoresist material or through the vias created in the photoresist material. After the polyamic acid is etched, the unexposed photoresist is removed, and the remaining polyamic acid is imidized, generally by heating, to form the final passivation coating. The vias (or openings) through the polymer coating permit access for electrical connections between the substrate and the outside environment.
Accordingly, the polyamic acid must be resistant to the solvent used to apply and process the photoresist material. Preferably, the polyimide is also resistant to the solvents that are often used in post-imidization processing, for example, the use of N-methylpyrrolidone (NMP) to clean the finished integrated circuit. On the other hand, the polyamic acid should be soluble in the wet-etch solution.
European patent application No. 284,803 (Published 10/5/88) teaches the preparation of siloxane polymers for microelectronic applications. This patent discloses fully imidized polyimides which are based on oxydiphthalic anhydride, a difunctional siloxane monomer, and an organic diamine that provides an asymmetrical structure in the polyimide siloxane polymer chain. The polyimide form of the polymer is soluble in NMP. Solubility in NMP is not a desirable attribute for a polyimide because NMP is generally used for post imidization procedures.
Davis et al., Recent Advances in Polyimide Science and Technology, Proc. 2nd Ellenville Conf. on Polyimides, 381-388 (1987), teaches a propyltetramethyldisiloxane, 3,3',4,4'-benzophenonetetracarboxylic acid dianhydride (BTDA), 4,4'-methylenedianiline (MDA) polyimide. There is no teaching that the compositions disclosed in this reference have a low modulus, nor any suggestion that the incorporation of siloxane will provide a low modulus polyimide composition. Further, there is no teaching that the composition provides a polyamic acid that is resistant to the solvents used with the photoresist material or in the wet-etching process. Rather, this reference notes that care must be taken in removing the photoresist material since the disclosed polyamic acid is prone to crazing in conventional photoresist developers.
While these references disclose siloxane-containing polymers for microelectronic applications, they do not disclose the invented siloxane-containing polyimides or the combination of low modulus, solvent resistance, and wet-etchability that are provided by the present invention.
Accordingly, it is an object of the present invention to provide an improved polyimide composition for microelectronic applications. Another object of the present invention is to provide a polyimide composition which exhibits a low modulus. A further object of the present invention is to provide a polyimide composition which is wet-etchable and is resistant to solvents generally used for photoresist processing. A still further object of the present invention is to provide polyimide compositions having advantageous properties. These and other objects and advantages of the present invention will be apparent to those skilled in the art from the following description.